1. Field of Invention
This invention relates to a panel instrument, such as mounted on a panel; and more particularly, to such panel instrument of reduced size.
2. Description of Related Art
A panel instrument, for example, a temperature controller, has a plurality of switches and displays on the front surface thereof, and has formed at the rear thereof a plurality of terminal boards, which are connected to internal printed circuit boards. Examples of such panel instruments are disclosed, for example in Japanese Laid Open Applications UM SN H3-124,692 (1991); H5-79,435 (1993); H5-87,524 (1993); H6-45,338 (1994); Japanese Patent H7-209,095 (1995); Japanese U.M. H7-16,975 (1995); H7-25,625 (1995); and H7-43,612 (1995). With the advance of technology, the size of the panel instrument has become smaller. However, impeding the reduction of size of such panel instruments are such factors as discussed below.
Display
In panel instruments, the visibility of display numerals, and the like, is not very good and so an enlarged display thereof is needed. However, currently, there is a limit on the size of the numerals and symbols being displayed. To provide enlarged symbols and the like for display, the case itself must also be enlarged. Also, since 7-segment display modules are of large vertical dimension, the height of the entire display being determined corresponding to the height of the 7-segment display module is also large, even though the height of the switches, and the like, is not great. Consequently, this factor interferes with the reduction in size of the panel instrument.
Waterproof and Dustproof Properties
Finger guards are usually provided for case terminals to satisfy safety standards. Such an arrangement is shown, for example, in Japanese UM H3-124,482(1991). However, in most cases, the mounting area is limited due to the presence of the finger guards so that reduction in size is impeded.
Thus, it is desirable to eliminate finger guards and yet still comply with safety standards. The prevailing safety standards require that the front cover cannot be removed manually and that the knob covers be made to be of small size. If, however, the knob covers are made smaller, the knobs cannot be readily moved because the rubber packing usually used therewith does not deform readily. Thus, if the knobs are forced, there is likely to be breakage of the rubber packing. In that case, waterproof and dustproof properties may become degraded.
Terminal Board
Even if the panel instrument is made smaller, the same number of terminals must be provided. Since usually one of the terminals has a thermocouple connected thereto for temperature measurement, temperature compensation is provided by measuring the terminal temperature using a sensor. Thus, space must be provided for such sensor which causes reduction in space for the other terminals. Also, since the temperature measured by the sensor is often different from the actual terminal temperature, errors may occur in the temperature measured by the thermocouple.
Moreover, with the reduction in size of the panel instrument, the density of electronic components mounted on the printed circuit boards (also know as "cards") is increased. Thus, the same number of boards or cards cannot be used as terminal rows at the rear because of the large dimensioned components, such as transformer and/or capacitors.
In order to improve the foregoing situation, it was proposed that the cards be connected to terminal rows and be disposed parallel to a card on which components were densely mounted using studs and that the electrical connections be made with connectors. However, the space problem was not entirely solved because the studs and connectors required added space, and such added space requirement raised more problems than solved, because size still could not be reduced.
Furthermore, since crimp-on terminal lugs are connected to the terminals of the panel instruments, clearance must be provided between the terminal rows to prevent the terminal lugs from touching the terminal screws. For this reason, reducing the size of the panel instruments in the prior art, required a limit on the number of terminals, which itself again produced more problems than solved.
In addition, since wiring, or the like, may sometimes be performed at the rear of a panel, as a safety measure, it is necessary to attach a cover which does not touch the terminals of the panel instruments. Mounting a terminal cover on the rear of the panel instrument with screws decreases the space for the mounting terminals. In turn, this prevents use of the required number of terminals for a large panel instrument, which factor impedes the reduction of the size of the panel instrument.
Heat Dissipation
Reduction of size of the panel instrument increases correspondingly, the heat dissipation per unit area. This necessitates the provision of heat radiation holes on the top surface of the panel instrument. However, the air surrounding the panel instrument may be surrounded with dust or the like. Thus, use of air holes may cause more problems, such as contamination of the inside with the dust, electrical short circuiting and fire, clogged air holes from the dust, degradation of heat dissipation,and reduced life of the electronic components.